Compared to other printing methods, the inkjet recording method offers ease of full-color printing since it involves a simpler process. And also, the inkjet recording method has the benefit of enabling the production of high-resolution images despite being of simple constitution.
As inkjet recording inks, dye-based inks are used in which various types of water-soluble dyes are dissolved in water or a mixture of water and an organic solvent. Such dye-based inks have a disadvantage of having poor light resistance, though they are excellent in color tone sharpness. Meanwhile, pigment-based inks in which carbon black and various types of organic pigments are dispersed are superior in light resistance compared to dye-based inks, and thus studies on pigment-based inks are being intensively made.
However, pigment-based inks are more likely to cause nozzle clogging compared to dye-based inks. Pigment inks are generally prepared as follows. A coloring material and a dispersant are preliminarily dispersed in an aqueous solvent to prepare a dispersion product. After that, the dispersion product is dispersed to a predetermined degree by a media-type disperser, followed by diluting to a predetermined concentration.
Water-based pigment inks usually contain a surfactant and a water-soluble resin to disperse a hydrophobic pigment therein, but the reliability of images obtained using such inks is extremely low. Thus, according to some disclosed techniques, film-formable resin fine particles are added into the inks in aim of improving the quality of images. However, it is difficult to keep a plurality of components in a state of being finely and stably dispersed throughout the ink for a long period of time. And so, when a dispersant such as a surfactant is used in a large amount in order to stably disperse such fine particles, a problem inconveniently occurs. Specifically, the problem, for example, is that air bubbles may be generated inside the ink tank and/or inkjet head, which could cause degradation in image quality. Likewise, in aim of improving dispersibility, research has been conducted into the effectiveness of methods that change the surface of a pigment to a hydrophilic group, or, the effectiveness of employing a resin containing a hydrophilic group. However, while a given component may be stable when used alone, combined use of several different components has problematically caused degradation of dispersion stability as well as degradation of storage stability.
PTLs 1 to 3 propose a surface-treated pigment ink, which gives excellent printing quality, and has excellent jetting stability and storage stability, or has only excellent jetting stability. PTL 4 proposes an ink set, which can maintain stable jetting performance of the ink.
To obtain high image density of a printed image, PTL 5 discloses a method in which an ink containing a water-insoluble coloring agent and charged resin quasi-particles which are smaller than particles of the coloring agent is used; PTL 6 discloses a method in which an ink containing a self-dispersible pigment whose DBP oil absorption is optimized is used; and PTL 7 discloses a method in which an aqueous dispersion liquid of carbon black containing carbon black which has been surface-modified to have HLB value of 7 to 18, and a nonionic surfactant having an acetylene skeleton in the molecular structure thereof is used. Moreover, PTL 8 discloses an aqueous ink containing an acetylene glycol-based surfactant.
For the purpose of stabilizing a dispersion state of an ink, PTL 9 discloses a method of dispersing a water-dispersible resin having a carboxyl group and a nonionic hydrophilic group in a molecule thereof in water; PTL 10 discloses a method in which polarities of a water-soluble polymer and a surfactant are both made the same, or nonions are added thereto; PTL 11 discloses a method an aqueous recording liquid is controlled so that an ionic coloring agent-containing polyester resin and hydrophilic groups of a coloring agent have the same polarity; and PTL 12 discloses a method in which a dispersed pigment and dispersed resin particles are controlled to have the same polarity. Moreover, PTL 13 discloses a printing ink using a gemini surfactant as a dispersing agent.
Furthermore, it is described in PTL 14 about an aqueous inkjet ink composition containing a pigment dispersion liquid, an aldehyde naphthalene sulfonate dispersing agent, and/or at least one sulfone solvent, where the pigment particle dispersion liquid contains pigment particles whose particle size distribution is such that at least 70% of the particles in the dispersion liquid has the diameter of less than 0.1 μm, and other particles have the diameter equal to or more than 0.1 μm. PTL 15 proposes a recording liquid formed of an aqueous medium containing a pigment, a polymeric dispersing agent and a nonionic surfactant.
Moreover, it is proposed in PTLs 16 and 17 that an AB or BAB block-copolymer is used as a dispersing agent of a pigment. It is disclosed in PTL 18 that a specific pigment, water-soluble resin and solvent are used.
As a method for dispersing a pigment without using a dispersing agent, PTL 19 discloses a method in which a substituent including a water-soluble group is introduced to carbon black; PTL 20 discloses a method in which a water-soluble monomer or the like is polymerized onto a surface of carbon black; PTL 21 discloses a method for acid-treating carbon black; and PTL 22 discloses a method in which water-resistance and jetting stability are attained by using an ink containing an acid-treated carbon black, and a terpolymer formed of acrylic acid, styrene, and α-methyl styrene.
Moreover, PTL 23 proposes an inkjet recording liquid containing dispersed particles whose average particle diameter is 30 nm to 200 nm.
Although the conventional ink liquids have succeeded to provide high image density with a color pigment ink, a black pigment ink has not achieved to provide sufficient image density and satisfactory black pigment inks have not been provided in this regard.
PTLs 24 to 26 disclose proposes an example in which beads each having a diameter of approximately 0.05 mm to approximately 1.0 mm are used for bead mill dispersion, but a dispersion stability of a resulting dispersion is not sufficient.
Moreover, PTL 27 discloses that an anionic surfactant is used as a dispersing agent, and that a molecular weight of such surfactant is preferably 1,000≦m≦30,000. However, a resulting ink liquid does not have sufficient dispersion stability, and there are problems such that the ink liquid loses its dispersion stability in the case where a pigment that is not resistant to impacts caused during dispersion is used, and as a result the ink liquid has poor jetting stability.